The early events involved in the process of T and B lymphocyte activation induced by nonspecific mitogens has been investigated. Using an artificial bilayer lipid membrane (BLM) it was found that one subclass of B cell mitogens, namely, large proteins such as Keyhole Limpet Hemocyanin (KLH) and polymer flagellin (POL) acted as channel-forming ionophores. Two known channel-forming ionophores, Excitibility Inducing Material(EIM) and the enzyme catalase were found to be B-cell specific mitogens. These findings suggest that some B-cell mitogens act by inducing cation-permeable channels in lymphocyte membranes leading to cell activation. T-cell activation induced by phytohemagglutinin (PHA) has been found to require a soluble factor from macrophages, lymphocyte activating factor (LAF). The reducing agent, 2-mercaptoethanol (2-ME) will enhance T-cell responses to PHA but requires the presence of small numbers of macrophages. 2-ME acts synergistically with LAF. Evidence suggests different mechanisms of action of these two compounds.